The present invention relates to a magnetic disk apparatus applicable to, for example, data processing equipment and implemented by a sector servo system. More particularly, the present invention is concerned with a magnetic disk apparatus capable of controlling the output timing of control current data by use of a servo estimation type observer.
A magnetic disk apparatus of the type described is disclosed in, for example, "Digital Control of Dynamic Systems--2nd ed." authored by Gene F. Franklin, J. David Powell and Micheal L. Workman and published by Addison-Wesley Publishing Company, Inc. (1990). This document teaches a closed-loop estimator made up of a plant system and an observer system. A magnetic disk apparatus implemented with such an estimator decodes servo information read out of a servo sector at a time t, converts control data calculated by an observer by using an equation (1), which is shown below, to an analog control current output by a digital-to-analog (D/A) converter, and then feeds the control current output to an actuator included in a positioning mechanism at a time t+1. EQU X(k+1)=.PHI.X(k)+.GAMMA.u(k)+Lp[y(k)-HX(k)] (1)
Specifically, it has been customary with a magnetic disk apparatus to decode servo information read out of a given servo sector, calculate control current data meant for the next sector by a servo estimation type observer the based on the equation (1), convert the calculated data to analog control current output by a D/A converter, and then feed the control current output to an actuator after servo information read out of the next servo sector has been decoded (see FIG. 4).
The problem with such a conventional magnetic disk apparatus is as follows. Since servo information are sequentially recorded at constant intervals in the recording surface of a magnetic disk medium, the sampling interval is too broad to secure a sufficient phase margin in relation to a frequency characteristic particular to the positioning mechanism. As a result, the operation of the positioning mechanism is not stable.